The invention is directed to metalized azo pigments, and particularly to process for preparing metalized azo pigment having superior color and viscosity stability in aqueous coating compositions.
Metalized azo pigments of the type prepared by coupling 3-hydroxy-2-naphthoic acid with the diazo of either 2-chloro-5-amino-toluene-4-sulfonic acid or 2-chloro-4-amino-toluene-5-sulfonic acid followed by precipitation with calcium, barium, strontium or manganese salts for a well-known class of colored pigments. The calcium azo pigment, which commonly contains strontium as a coprecipitant, is widely used in printing inks because of good bleed resistance, stable viscosity, high color intensity, and good dispersibility.
The earliest preparations of these metalized azo pigments is described in U.S. Pat. Nos. 1,803,657, 2,117,860, and 2,225,665. In all cases, the basic procedure for preparing metalized azo pigment involves the steps of diazotization and coupling, followed by precipitation with a metal salt. A variation of the basic procedure was later described in U.S. Pat. No. 2,744,027. This procedure involves mixing a predetermined amount of azo dye formed from diazotized 2-chloro-4-amino-toluene-5-sulfonic acid and 3-hydroxy-2-naphthoic acid with the azo dye from 2-chloro-4-amino-anisole-5-sulfonic acid and 3-hydroxy-2-naphthoic acid prior to precipitation with a metal salt.
The calcium azo pigments prepared by the above procedures are satisfactory for a wide variety of applications, but undergo an undesirable hue change from a blue shade red to a yellow shade when aged in aqueous ink systems. Historically this color drift has been a problem since the first significant commercial use of aqueous inks in the 1950's. However, little attention was paid to this problem at the time, because of the relatively low volume of aqueous ink used. In recent times growing concern over the environment has caused ink manufacturers and users to increasingly prefer aqueous inks to avoid the pollution problems associated with organic inks. Since calcium azo pigment is unsuitable in aqueous inks, the industry has turned to inferior substitutes which provide color stability, but lack the desirable lightfastness of the calcium azo pigment.
This invention provides an improved process for preparing calcium azo pigment whereby the resulting pigment retains the desired properties such as lightfastness and retains color and viscosity stability in aqueous compositions.